Ni—Cr alloys such as Alloy600 and Alloy690 are employed for steam generator (SG) tubes for use in nuclear power plants. This is because these alloys have excellent corrosion resistance in a high temperature water environment. However, when even a trace amount of a metal component is released due to corrosion and is radioactivated in a plant, such a radioactivated metal component functions as a radiation source. Therefore further improvement of the corrosion resistance is desired.
Conventional arts to form a protective oxide film on the surface of a heat transfer tube aiming at suppression of the exposure to radiation have been proposed. For example, Patent Document 1 discloses a method in which a Ni-based alloy tube is heat treated at a pressure of 10−2 to 10−4 Torr and at a temperature of 400 to 750° C. in order to form an oxide film mainly composed of chromium oxide. According to this method, the general corrosion resistance is improved.
Patent Document 2 discloses a method for producing a member for a nuclear power plant in which method a Ni-based precipitation strengthened alloy is solution heat treated, and then heat treated in an oxidizing atmosphere of 10−3 Torr to atmospheric pressure for the purpose of an age hardening treatment and an oxide film formation treatment where one treatment combine at least a part of another treatment. Additionally, Patent Document 3 discloses a method for producing a Ni-based alloy product in which method the Ni-based alloy product is heat treated in an atmosphere of hydrogen or a mixed gas of hydrogen and argon with the dew point of −60 to +20° C.
Patent Document 4 discloses a method for forming a chromium enriched layer by exposing an alloy workpiece containing Ni and Cr to a gas mixture composed of water vapor and at least one nonoxidizing gas. Additionally, Patent Document 5 discloses a method for producing a Ni-based alloy in which method a Ni-based alloy is heated in an atmosphere containing carbon dioxide gas to form an oxide film composed of chromium oxide on the surface of the Ni-based alloy.
Citation List
Patent Document
Patent Document 1: JP64-55366A
Patent Document 2: JP8-29571A
Patent Document 3: JP2002-121630A
Patent Document 4: JP2002-322553A
Patent Document 5: JP2006-111902A